In recent years, a non-destructive sensing technique using a terahertz wave has been developed. As an application field of the electromagnetic wave of this frequency band, there is an imaging technique for a safe tomography apparatus instead of an X-ray apparatus. Examples of the tomographic imaging method include a method in which an electromagnetic pulse is transmitted to a sample, and the reflected time waveform is measured to calculate the distance between two interfaces of the sample from the time difference of each electromagnetic pulse reflected on the two interfaces (Time-of-Flight method).
An example of the tomographic imaging method includes a technique disclosed in Non Patent Literature 1. The Non Patent Literature 1 discloses a technique for imaging a coating of a tablet using a terahertz pulse. According to the technique of the Non Patent Literature 1, a terahertz pulse emitted to a coated tablet is reflected on an interface between air and the coating and an inside interface between the coating and the tablet. As a result, the measured time waveform of a reflected terahertz wave includes two peaks corresponding to the two interfaces. Assuming that At denotes the time interval between the two peaks, the coating width d can be expressed by the following expression.2d =Δtc/n where c denotes the speed of light in a vacuum, and n denotes the refractive index of the coating material. The measured coating width is about several 10 μm. The Non Patent Literature 1 discloses an example of measuring the correlation between the coating width calculated by the aforementioned method and the drug release rate.